Plate tectonics began early in Earth's history

Boundary of North American and Eurasian plates seen along the Mid-Atlantic Ridge in Iceland: Westward view over the top of the east edge of the North American tectonic plate.Courtesy Joe HatfieldGeochemists at UCLA have determined plate tectonics – the theory involving the movement and collision of crustal plates – began much sooner after the Earth’s formation 4.5 billion years ago than thought previously.

Until now, plate tectonics were thought to have begun around 350 million years ago - or even later – but this new UCLA data points to much earlier beginnings.

"We are proposing that there was plate-tectonic activity in the first 500 million years of Earth's history," said professor Mark Harrison, director of UCLA's Institute of Geophysics and Planetary Physics and co-author of the paper. "We are reporting the first evidence of this phenomenon."

The theory of moving crustal plates floating on molten rock was first championed in Alfred Wegener’s 1912 work, The Origins of Continents and Oceans. Wegener suggested all the continents existing today originated in a super-continent he called Pangaea that spread apart over time due to “continental drift”. Most scientists were skeptical of the theory until the 1960s when it was bolstered by the discovery of sea floor spreading.

Harrison and his colleagues analyzed zircon crystals found in 3 billion year old rocks from Western Australia. The rocks were formed from ancient magmas that had cooled and froze the mineral crystals in place. Using an ion microprobe, they bombarded the zircon with a beam of charged atoms (ions). The bombardment caused the zircon crystals to release their own ions and these were then analyzed using a mass spectrometer. The analysis showed the zircon crystals were more than 4 billion years old. It also showed the zircons had formed in an area where the heat flow was much lower than expected.

"The global average heat flow in the Earth's first 500 million years was thought to be about 200 to 300 milliwatts per meter squared," said Michelle Hopkins, a UCLA graduate student in Earth and space sciences, and the study's lead author. "Our zircons are indicating a heat flow of just 75 milliwatts per meter squared — the figure one would expect to find in subduction zones, where two plates converge, with one moving underneath the other."

The only places on Earth today where the average heat flow is one third that of the rest of the planet are in convergent plate-tectonic boundaries where magmas are forming.

Harrison published an earlier study in 2001 proving water was present early on the surface of the Earth during its formative years, and this current data strengthens his claim because plate tectonics can’t occur on a dry planet.

All this new information forces scientists to reevaluate their conception of how Earth appeared early in its formation.

"Unlike the longstanding myth of a hellish, dry, desolate early Earth with no continents, it looks like as soon as the Earth formed, it fell into the same dynamic regime that continues today," Harrison said. "Plate tectonics was inevitable, life was inevitable. In the early Earth, there appear to have been oceans; there could have been life — completely contradictory to the cartoonish story we had been telling ourselves."

Plate tectonics are responsible for quite a few phenomena, including but not limited to earthquakes. The really cool thing about this research is that it suggests that Earth wasn't a dry, hellish place during the earliest years of our planet. It was a place with water, which made possible the same kinds of rock processes that we see going on today. And that contradicts a bunch of previous scientific thinking about the evolution of our planet and the life dependent on it.

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